Persist values, a11y errors, and aria-invalid on /register

- Server action now echoes firstName, lastName, email, and the privacy
  checkbox state on every error path so failed submits don't wipe the form.
- New `attempt` counter on RegisterState drives a remount key per field,
  bypassing React's default <form action> reset semantics.
- Error <p> elements gain role="alert" + aria-live="polite" so screen
  readers announce validation failures live.
- Privacy checkbox now flags aria-invalid when not accepted and points to
  the error message via aria-describedby.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/register/actions.ts

@@ -32,6 +32,16 @@ const registerSchema = z.object({
 const PRIVACY_POLICY_VERSION = "2026-05-01";
 
 export type RegisterState = {
+  // Bumped after every action call so the client can key uncontrolled inputs
+  // and force a remount with the echoed values from a failed submit.
+  attempt: number;
+  // Echoed inputs (sans password) so failed submits don't wipe the form.
+  values?: {
+    firstName?: string;
+    lastName?: string;
+    email?: string;
+    acceptPrivacyPolicy?: boolean;
+  };
   errors?: {
     email?: string[];
     password?: string[];
@@ -42,13 +52,29 @@ export type RegisterState = {
   };
 };
 
+export const initialRegisterState: RegisterState = { attempt: 0 };
+
+function echoValues(formData: FormData): RegisterState["values"] {
+  return {
+    firstName: String(formData.get("firstName") ?? ""),
+    lastName: String(formData.get("lastName") ?? ""),
+    email: String(formData.get("email") ?? ""),
+    acceptPrivacyPolicy: formData.get("acceptPrivacyPolicy") === "on",
+  };
+}
+
 export async function registerAction(
-  _prevState: RegisterState,
+  prevState: RegisterState,
   formData: FormData,
 ): Promise<RegisterState> {
+  const attempt = prevState.attempt + 1;
   const parsed = registerSchema.safeParse(Object.fromEntries(formData));
   if (!parsed.success) {
-    return { errors: parsed.error.flatten().fieldErrors };
+    return {
+      attempt,
+      values: echoValues(formData),
+      errors: parsed.error.flatten().fieldErrors,
+    };
   }
   const { email, password, firstName, lastName } = parsed.data;
 
@@ -77,6 +103,8 @@ export async function registerAction(
       err.code === "P2002"
     ) {
       return {
+        attempt,
+        values: echoValues(formData),
         errors: {
           email: ["Mit dieser E-Mail-Adresse existiert bereits ein Account."],
         },
@@ -93,5 +121,5 @@ export async function registerAction(
   });
 
   // Unreachable – signIn redirected.
-  return {};
+  return { attempt };
 }